Gas-fired natural draft appliances, such as the typical water heaters and furnaces in American homes, are equipped with a draft hood which protects the combustion process from excess draft and from backdraft. Excess draft in the vent draws ambient air through the draft hood. In case of backdraft or no draft conditions the flue gases flow from the heat exchanger through the draft hood out of the appliance. Without a draft hood a backdraft would cause incomplete combustion (excessive CO.sub.2 and CO) and might blow out the flame.
If the appliance is installed in a home and draws conditioned air for combustion the vent acts like an open window during the standby periods, allowing warm room air to escape, or cold outside air to enter. Automatic vent dampers reduce this loss by closing the vent during the standby periods of the heating cycle.
Thermally controlled vent dampers have been used for this purpose for many years. They operate by sensing the temperature prevailing in the damper. In addition to their primary job of closing the vent during standby they adjust the vent opening during the operation of the appliance by flexing to a partially closed position if the bimetallic elements are cooled by a large amount of dilution air aspirated through the draft hood. This modulating function of thermally controlled vent dampers reduces heat loss during the operating period of the heating cycle.
While known designs of thermally controlled vent dampers reduce excessive dilution air to a varying degree they cannot optimize this function since they sense the temperature in the vent connector. At that point the vent gases constitute a mixture of the flue gases coming from the heat exchanger and the dilution air coming through the draft hood. The temperature of the flue gases varies widely from one appliance to the other and even in the same appliance during one heating cycle. The automatic adjustment of the amount of excess dilution air (and thereby the reduction of heat loss) to a practical minimum by sensing only the temperature of the flue gas/air mixture is therefore an approximation.